The increase of CO2 in the atmosphere is thought to have a major influence on global climate. It is therefore desirable that the emission of anthropogenic CO2 into the atmosphere is reduced. The capture and storage of CO2 provides a way to avoid emitting CO2 into the atmosphere, by capturing CO2 from sources such as oil and natural gas processing plants and power plants, transporting it and injecting it into deep rock formations. At depths below about 800-1000 m, CO2 is in a supercritical state that provides potential for efficient utilization of underground storage space. It is considered likely that injecting CO2 into deep geological formations at carefully selected sites will enable storage for long periods of time with a predicted 99% of the CO2 being retained for 1000 years. CO2 can remain trapped underground by virtue of a number of mechanisms, including trapping below an impermeable, confining layer (caprock), retention as an immobile phase trapped in the pore spaces of the storage formation, dissolution in in situ formation fluids, absorption onto organic matter in coal and shale and by reacting with minerals in the storage formation and caprock to carbonate materials. Suitable fields for long-term storage include depleted oil and gas reservoirs, saline formations, which are deep underground porous reservoir rocks saturated with brine and possibly coal formations.
An extensive review of existing CO2 Capture and Storage (CCS) projects and technology is given in the IPCC Special report on Carbon Dioxide Capture and Storage (CCS) (“Carbon Dioxide Capture and Storage”, IPCC, 2005, editors: Metz et al., Cambridge University Press, UK; also available at: http://www.ipcc.ch). The paper SPE 127096 “An overview of active large-scale CO2 storage projects”, I. Wright et al. presented at the 2009 SPE International Conference on CO2 capture, Storage and Utilization held in San Diego, Calif., USA 2-4 Nov. 2009 provides a more recent update on existing large-scale CO2 storage projects. Of the commercial scale projects reviewed in these documents, the most significant in terms of cumulative volume injected are the Sleipner and In Salah projects.
US 20100116511 A1 discloses a method for permanent storage of CO2 compositions in a subterranean geological formation, where the conditions are continuously adapted such that the composition is injected in supercritical state. Said document also describes an arrangement for injection of CO2 in the formation, which consists of a conduit having an injection port and means for control of the injection parameters with the possibility of changing the parameters.
The Sleipner CGS Project is located 250 km off the Norwegian coast and is operated by StatoilHydro. The CO2 is stored in supercritical state in the Utsira formation at a depth of 800-1000 m below the sea surface. CO2 produced during natural gas processing is captured and subsequently injected underground. CO2 injection started in October 1996 and by 2008, more than ten million tons of CO2 had been injected at a rate of approximately 2700 tons per day. A shallow long-reach well is used to take the CO2 2.4 km away from the producing wells and platform area. The injection site is placed beneath a local dome of the top Utsira formation.
The In Salah CCS Project is an onshore project for the production of natural gas located in the Algerian Central Sahara. The Krechba Field produces natural gas containing up to 10% of CO2 from a number of geological reservoirs. CO2 is stripped from the gas and re-injected into a sandstone reservoir at a depth of 1800 m enabling the storage of 1.2 Mt of CO2 per year.
While the global capacity to store CO2 deep underground is believed to be large, the opening of a new storage site is inevitably costly as it requires an assessment of potential risk to humans and the ecosystem. It is thus desirable that existing sites are exploited to maximum capacity. Yet current estimates suggest that the existing mechanisms used to inject supercritical CO2 into deep storage sites result in only around 2% of the pore volume of the geological storage site being utilized for CO2 sequestration. This is believed to be due to the uneven sweep of the injected CO2 in subterranean formations, which leads to a phenomenon called “fingering” in which the CO2 injection front is highly uneven with small areas of high penetration surrounded by areas in which the CO2 has not penetrated at all. Pursuing current practices will result in the loss of considerable storage volume in available storage sites.
In view of this state of the art it is an object of the present invention to provide an improved method for the permanent storage of CO2 in subterranean geological formations.
It is a further object of the present invention to provide methods which allow for a more efficient use of the storage capacity of geological formations, for permanent storage of CO2.